High speed envelope manufacturing machines must meet the demands to produce large quantities of high quality, envelopes in an inexpensive manner. These envelope machines have the capability of producing envelopes at speeds well in excess of 1,000 envelopes per minute and are typically microprocessor controlled high speed web machines which are designed to perform every aspect of envelope manufacturing, beginning with the unwinding of a continuous web of fibrous material. The fibrous material is then fed through the envelope machine where it may be printed with desired information, provided with an adhesive material, cut to create a window area, provided with a transparent window for the window area, cut to form individual envelope units, and folded over a desired package as to create a stuffed envelope.
A shortcoming of the aforementioned envelope manufacturing machines is that they must operate at a relatively slow rate when producing envelopes with windows. That is, the speed is limited by the speed at which a window area can be created in the fibrous material and by the rate at which a transparent window can be placed overtop the window area. A manufacturer will therefore have to choose between producing envelopes at a slower rate and producing envelopes that do not contain a window. However the envelope windows are beneficial in that they allow for the display of information about the contents of the envelope or about the individual to whom the envelope will be sent, such as the recipient's address.
To increase the rate of the envelope manufacturing process, the roll of fibrous material may include pre-cut openings and transparent windows. However, the roll of fibrous material consists of thousands of envelope units and will therefore contain thousands of transparent windows. These transparent windows typically have a thickness on the order of 1.25*10−3 inches, which will add to the total thickness of the envelope unit at the areas in which the window cover is placed over top of the fibrous material.
A web of fibrous material with transparent windows will therefore have a non-uniform thickness. When this web is wound into a roll, it will have an asymmetrical diameter. Given the size and weight of the roll and the speed at which the roll is unwound during the envelope making process, any asymmetry in the roll will cause the unwinding process to be uneven and erratic. In turn, this can cause shuddering or shaking in the envelope machine, which can result in damage to the machine or fibrous material. For this reason, it is important that the cylinder of fibrous material have a circular cross-section when it is wound.